Methane gas sensor

ABSTRACT

An apparatus and method for detection of methane in an environment, including a housing with a sensor printed circuit board and a processor printed circuit board interconnected to and thermally insulated from the sensor printed circuit board. The sensor printed circuit board includes a first tuning fork isolated from the environment, and a second tuning fork exposed to the environment. The tuning forks are attached to opposite sides of the sensor printed circuit board. The processor printed circuit board includes processing circuitry interconnected to the first tuning fork and the second tuning fork, which receives vibration frequency signals therefrom, and is programmed to determine a frequency difference between the frequency of vibration of the first tuning fork and the frequency vibration of the second tuning fork, and if the frequency difference is greater than a predetermined threshold, then setting an alarm to indicate the presence of methane.

This application claims the benefit or priority pursuant to 35 U.S.C.119(e) from U.S. Provisional Patent Application having Application No.62/631,756 filed on Feb. 17, 2018.

TECHNICAL FIELD

This invention relates generally to the detection of methane gas, and inparticular to a methane/natural gas sensor for residential applications.

BACKGROUND

Currently available methane detectors have a problem with false alarms,where the sensors are sensitive to not only methane but also to othermaterials in the air such as hairspray. As a result of such falsedetection, present day detectors are unreliable, and the presentinvention solves this problem as described herein.

SUMMARY

Provided herein is a methodology for using two tuning forks as part of adevice, where one of the tuning forks is isolated from the environmentthat is being monitored. This control tuning fork will be within avacuum and it will be heated so that temperature and pressure arecontrolled. The second (isolated) tuning fork is exposed to the ambientenvironment so that if methane is detected in the environment, acomparison is made of the frequency of vibration between theambient-exposed tuning fork and the isolated tuning fork. By sensing thedifference between the frequency of vibrations of the two tuning forks,the present invention determines if methane is present and causes analarm to go off.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of the circuitry of the methane detector ofthe preferred embodiment.

FIG. 2 is a detailed schematic of the methane sensor sub-circuit of themethane detector of FIG. 1.

FIG. 3 is a detailed schematic of the pressure sensor sub-circuit of themethane detector of FIG. 1.

FIG. 4 is a detailed schematic of a temperature sensor sub-circuit ofthe methane detector of FIG. 1.

FIG. 5 is a detailed schematic of the humidity sensor sub-circuit of themethane detector of FIG. 1.

FIG. 6 is a detailed schematic of the heater sub-circuit of the methanedetector of FIG. 1.

FIG. 7 is a perspective illustration of the methane sensor in a plastichousing (fully assembled).

FIG. 8 is an exploded view of the methane sensor of FIG. 7.

FIG. 9 is a flowchart of the methane concentration calculationalgorithm.

FIG. 10 is a graph of the typical response of the tuning fork oscillatorto pressure changes in the preferred embodiment.

FIG. 11 is a graph of the typical response of the tuning fork oscillatorto methane in the preferred embodiment.

FIG. 12 is a graph of the typical response of the tuning fork oscillatortemperature variations in the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The operation of the sensor is based on the assessment of the molar massof a gas mixture wherein the effective molar mass of the monitored gasdecreases in the presence of low mass methane molecules. The lower molarmass of the monitored gas mixture results in higher frequency ofoscillations of a quartz resonator open to the monitored environment,such as a quartz tuning fork.

A pressure sensor is used to compensate against atmospheric pressurevariations and calculate the correct molar mass of the gas mixture. Theoscillation frequency of the open resonator shifts with gas pressurechanges as well as with the variations in the methane concentration inair within the methane concentration range from 0% to 100%.

A second quartz oscillator can be used as a reference element whereinthis second oscillator can be open to an atmosphere which does notcontain methane, or the said oscillator can be vacuum sealed bymanufacturer. In this case, a differential frequency between the twooscillators is a function of the methane concentration in a gas mixture.

In a preferred embodiment, the differential frequency can be obtainedusing a D flip-flop trigger. Signals from the two oscillators areconnected to the two inputs (D input and Clock input) of the trigger.The trigger output producing the differential frequency signal is a lowfrequency signal. This signal modulates an external continuous highfrequency signal (carrier frequency), and the number of the pulsesmodulated by the sensor differential frequency is counted. The number ofpulses can be counted every second. The number of pulses is a nearlylinear function of the methane concentration.

To improve the sensor stability against temperature variations, thequartz tuning fork resonators are kept at a constant temperature usingexternal heating elements. Temperature sensing elements such asthermistors can be used in the temperature control loop, while theheating elements can be resistors mounted next to the oscillators.

Since the oscillators and the thermistors have physically differentlocations, it is possible that the second order temperature deviationsmay occur in the oscillator if the ambient temperature changes. However,it is safe to assume that these small oscillator temperature variationsare linear functions of the ambient temperature within a workingtemperature range of the methane sensor.

As the thermistors are used in the temperature control loop, theirtemperature (or control resistance) is always fixed. In order to assessthe actual oscillator temperature deviations, a heating power or asensor heater PWM duty factor can be used for the temperaturecompensation.

In order to further improve the sensor stability in differentenvironments, an external humidity sensor can be used to compensateagainst variations in relative humidity. However, in normal operationconditions inside residential buildings the use of the humidity sensormay not be needed.

Thus, knowing that the sensor signal depends essentially linear on theambient pressure, temperature, and the methane concentration, it ispossible to use an easy for computation first order concentrationcalculation formula such as below:

C=[(dF−D)*(101325/p)+c1*PWM+c2*RH]/S,  [1]

where C is the methane concentration, dF is the number of pulses, p isatmospheric pressure in kPa, PWM is the heater duty factor, RH is therelative humidity, S is the sensor span, D is a parameter accounting forthe sensor response to pressure and the sensor drift, and c1 and c2 arecalibration coefficients.

The unknown coefficients as well as the sensor span are obtained thoughtthe sensor calibration. The calibration procedure consists of at leasttwo-point sensor tests such as at two different temperatures, twodifferent pressures, and two different concentrations, wherein one ofthe set points can be a room temperature, an atmospheric pressure, and0% methane concentration.

To further simplify the test procedure, it is possible to use thesensitivity of the methane sensor to pressure variations fordetermination of the sensor sensitivity to methane (sensor span). Fromthe formula [1], we can see that

S˜k*dF/dp,  [2]

where k is a constant.

The sensor drift is due to slow oxidation of the quartz resonatorelectrodes and resonator surface coating with dust, tar, or chemicals.In order to compensate for the slow drift, an auto-zero function can beimplemented in the sensor, such as a high pass filter. In a preferredembodiment, the high pass filter is implemented in the sensor firmware,it can transmit signals with the time constant on the order of hours andreject slow sensor drift with a time constant on the order of days.

In a preferred embodiment, the methane sensor comprises two oscillatorseach of which have a quartz tuning fork resonator. One of the two tuningforks is open to the environment, the other one is factory sealed. Thetwo oscillators are located next to each other, preferably on theopposite sides of a sensor printed circuit board (PCB). The sensorprinted circuit board having the oscillators is thermally insulated fromthe main processor circuit board by narrow contact bridges. The sensorprinted circuit board has SMD heating elements that are powered via thetemperature control loop to keep the temperature of the oscillators ataround 55 C.

To further improve the thermal insulation and minimize convectionlosses, the sensor PCB is enclosed within a porous thermally insulatingmaterial. The porous material is gas permeable, and it inhibits airconvection around the sensor PCB. The thermally insulating enclosurehelps decrease thermal losses and power consumption by the methanesensor.

The sensor is based on an MSP430 microprocessor, has a push button,three status LEDs, sound alarm, wall plug power supply, back-up battery,and is designed to conform the UL 1484 standard.

Reference is now made to FIG. 1, which is a block diagram of thecircuitry 100 of the methane detector of the preferred embodiment. Shownin FIG. 1 is a microcontroller 102 that is connected to a methane sensorsub-circuit 104 (see FIG. pressure sensor sub-circuit 106 (see FIG. 3),a humidity sensor sub-circuit 110 (see FIG. 5), a heater sub-circuit 108(see FIG. 6), LEDs 112, a buzzer 114, a button 116, RTDs 118, JTAG 120,UART (universal asynchronous receiver-transmitter) 122, and oscillator124.

FIG. 7 shows a perspective illustration of the methane sensor in aplastic housing, in fully assembled form, while FIG. 8 is an explodedview of the methane sensor of FIG. 7. Major components include a housing808, a base 810 that is affixed to the housing 808, and a PC board 812that is affixed to the base 810 as shown. The PC board 812 holds theelectronic components including for example the resonator under thermalinsulation 820, push-button 824, a buzzer 818 and several LEDs 822 (e.g.power and alarm indicators). This assembly is protected by the top cover814 and a battery cover 816 as shown.

FIG. 9 is a flowchart of the methane concentration calculationalgorithm. The frequency signal from the oscillator at step 902 is usedto modulate the carrier frequency at 904, which is then fed to the pulsecounter at 906. That, along with pressure sensor data at 908 and pulsewidth modulation duty cycle data at 910 are used (along with warm-up endsignal 916 and calibration coefficients (918) to calculate theconcentration every second at step 912. This is fed to high pass filterat 914, and then the baseline is calculated at 920. When data is readyat 922, it is checked to see if the concentration is greater than 1% at924. If Yes, then audio and visual alarms are triggered; if No, then theloop feeds back to step 912.

For reference purposes, FIG. 10 is a graph of the typical response ofthe tuning fork oscillator to pressure changes in the preferredembodiment; FIG. 11 is a graph of the typical response of the tuningfork oscillator to methane in the preferred embodiment; and FIG. 12 is agraph of the typical response of the tuning fork oscillator totemperature variations in the preferred embodiment.

The following invention is claimed:
 1. An apparatus for detection ofmethane in an environment, the apparatus comprising a housing comprisinga sensor printed circuit board and a processor printed circuit boardinterconnected to and thermally insulated from the sensor printedcircuit board by at least one contact bridge, the sensor printed circuitboard comprising a first tuning fork, the first tuning fork beingisolated from the environment, and a second tuning fork, the secondtuning fork being exposed to the environment, wherein the first tuningfork and the second tuning fork are attached to opposite sides of thesensor printed circuit board; the processor printed circuit boardcomprising processing circuitry interconnected via the contact bridge tothe first tuning fork and the second tuning fork and receiving vibrationfrequency signals therefrom, the processing circuitry being programmedto determine a frequency difference between the frequency of vibrationof the first tuning fork and the frequency vibration of the secondtuning fork, and if the frequency difference is greater than apredetermined threshold, then setting an alarm to indicate the presenceof methane.
 2. The apparatus of claim 1 wherein the processing circuitrycomprises a D flip-flop and a microcontroller, wherein the vibrationfrequency signals from the tuning forks are interconnected to the Dinput and clock input of the D flip-flop for determining thedifferential frequency, the trigger output of the D flip-flop isinterconnected to the microcontroller that counts pulses within acertain time period by modulating an external high frequency carriersignal; wherein the number of pulses counted is a nearly linear functionof the methane concentration.
 3. The apparatus of claim 1 wherein thefirst tuning fork and the second tuning fork each comprise a quartzoscillator.
 4. The apparatus of claim 1 further comprising a casing inwhich the first tuning fork is located, the casing being vacuum sealedto provide temperature and pressure isolation from the environment. 5.The apparatus of claim 4 further comprising a heating element mounted inproximity to the casing wherein the casing is heated by the heatingelement.
 6. The apparatus of claim 1 further comprising a pressuresensor, a humidity sensor, and a thermistor each mounted within thehousing and interconnected with the processing circuitry on theprocessor printed circuit board.
 7. A method for detection of methane inan environment, the method comprising providing in a housing a sensorprinted circuit board, and a processor printed circuit boardinterconnected to and thermally insulated from the sensor printedcircuit board by at least one contact bridge, providing on the sensorprinted circuit board a first tuning fork, the first tuning fork beingisolated from the environment, and a second tuning fork, the secondtuning fork being exposed to the environment, wherein the first tuningfork and the second tuning fork are attached to opposite sides of thesensor printed circuit board; providing on the processor printed circuitboard processing circuitry interconnected via the contact bridge to thefirst tuning fork and the second tuning fork wherein the processingcircuitry receives vibration frequency signals therefrom, determines afrequency difference between the frequency of vibration of the firsttuning fork and the frequency vibration of the second tuning fork, andsets an alarm to indicate the presence of methane if the frequencydifference is greater than a predetermined threshold.
 8. The method ofclaim 7 wherein the processing circuitry comprises a D flip-flop and amicrocontroller, the vibration frequency signals from the tuning forksare interconnected to the D input and clock input of the D flip-flop fordetermining the differential frequency, the trigger output of the Dflip-flop is interconnected to the microcontroller that counts pulseswithin a certain time period by modulating an external high frequencycarrier signal; and the number of pulses counted is a nearly linearfunction of the methane concentration.
 9. The method of claim 7 whereinthe first tuning fork and the second tuning fork each comprise a quartzoscillator.
 10. The method of claim 7 further comprising providing acasing in which the first tuning fork is located, the casing beingvacuum sealed to provide temperature and pressure isolation from theenvironment.
 11. The method of claim 10 further comprising mounting aheating element in proximity to the casing wherein the casing is heatedby the heating element.
 12. The method of claim 7 further comprisingmounting a pressure sensor, a humidity sensor, and a thermistor withinthe housing and interconnecting them with the processing circuitry onthe processor printed circuit board.